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            <h1 class="post-title">设计模式之设计原则-开闭原则 (OCP)</h1>
            
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                    <i class="iconfont icon-calendar"></i> 发表于 2020-02-05
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                                <a href="/categories/%E8%AE%BE%E8%AE%A1%E6%A8%A1%E5%BC%8F/"> 设计模式</a>
                            
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                                <a href="/categories/%E8%AE%BE%E8%AE%A1%E6%A8%A1%E5%BC%8F/"># 设计模式</a>
                            
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            <p><a href="https://emojipedia.org/backhand-index-pointing-right/" target="_blank" rel="noopener">👉</a><a href="https://github.com/zhaohaihao/java-design-patterns/tree/master/001-OCP" target="_blank" rel="noopener"><strong>文章示例代码</strong></a><a href="https://emojipedia.org/backhand-index-pointing-left/" target="_blank" rel="noopener">👈</a></p>
<h1 id="定义"><a href="#定义" class="headerlink" title="定义"></a>定义</h1><blockquote>
<p>一个软件实体如类、模块和函数应该对扩展开放，对修改关闭。</p>
</blockquote>
<p>开闭原则强调的是用抽象构建框架，用实现来扩展细节。</p>
<h1 id="场景示例"><a href="#场景示例" class="headerlink" title="场景示例"></a>场景示例</h1><p>笔者这里以超市中的商品做一个简单的示例来方便理解。</p>
<h2 id="创建商品接口"><a href="#创建商品接口" class="headerlink" title="创建商品接口"></a>创建商品接口</h2><p>首先创建一个商品接口，同时在接口内部定义获取商品ID、名称、价格的各个方法。一般来说，一个商品是一个实体，后续我们可以去写一个类包含商品ID、名称、价格这三个成员变量，这里仅仅为了演示开闭原则。</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">/**</span></span><br><span class="line"><span class="comment"> * <span class="doctag">@author</span> zhh</span></span><br><span class="line"><span class="comment"> * <span class="doctag">@description</span> 商品接口</span></span><br><span class="line"><span class="comment"> * <span class="doctag">@date</span> 2020-02-04 22:47</span></span><br><span class="line"><span class="comment"> */</span></span><br><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">interface</span> <span class="title">IGoods</span> </span>&#123;</span><br><span class="line"></span><br><span class="line">    <span class="comment">/**</span></span><br><span class="line"><span class="comment">     * 获取商品ID</span></span><br><span class="line"><span class="comment">     */</span></span><br><span class="line">    <span class="function">Integer <span class="title">getId</span><span class="params">()</span></span>;</span><br><span class="line"></span><br><span class="line">    <span class="comment">/**</span></span><br><span class="line"><span class="comment">     * 获取商品名称</span></span><br><span class="line"><span class="comment">     */</span></span><br><span class="line">    <span class="function">String <span class="title">getName</span><span class="params">()</span></span>;</span><br><span class="line"></span><br><span class="line">    <span class="comment">/**</span></span><br><span class="line"><span class="comment">     * 获取商品价格</span></span><br><span class="line"><span class="comment">     */</span></span><br><span class="line">    <span class="function">Double <span class="title">getPrice</span><span class="params">()</span></span>;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<h2 id="创建商品子类"><a href="#创建商品子类" class="headerlink" title="创建商品子类"></a>创建商品子类</h2><p>我们的商品其实是有很多分类的，比如食品、用品、电器等等。</p>
<p>下面笔者以食品为例，创建食品类的同时实现商品接口。</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">/**</span></span><br><span class="line"><span class="comment"> * <span class="doctag">@author</span> zhh</span></span><br><span class="line"><span class="comment"> * <span class="doctag">@description</span> 食品类</span></span><br><span class="line"><span class="comment"> * <span class="doctag">@date</span> 2020-02-04 23:42</span></span><br><span class="line"><span class="comment"> */</span></span><br><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">Food</span> <span class="keyword">implements</span> <span class="title">IGoods</span> </span>&#123;</span><br><span class="line"></span><br><span class="line">    <span class="keyword">private</span> Integer id;</span><br><span class="line"></span><br><span class="line">    <span class="keyword">private</span> String name;</span><br><span class="line"></span><br><span class="line">    <span class="keyword">private</span> Double price;</span><br><span class="line"></span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="title">Food</span><span class="params">(Integer id, String name, Double price)</span> </span>&#123;</span><br><span class="line">        <span class="keyword">this</span>.id = id;</span><br><span class="line">        <span class="keyword">this</span>.name = name;</span><br><span class="line">        <span class="keyword">this</span>.price = price;</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    <span class="function"><span class="keyword">public</span> Integer <span class="title">getId</span><span class="params">()</span> </span>&#123;</span><br><span class="line">        <span class="keyword">return</span> <span class="keyword">this</span>.id;</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    <span class="function"><span class="keyword">public</span> String <span class="title">getName</span><span class="params">()</span> </span>&#123;</span><br><span class="line">        <span class="keyword">return</span> <span class="keyword">this</span>.name;</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    <span class="function"><span class="keyword">public</span> Double <span class="title">getPrice</span><span class="params">()</span> </span>&#123;</span><br><span class="line">        <span class="keyword">return</span> <span class="keyword">this</span>.price;</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<h2 id="测试类及输出"><a href="#测试类及输出" class="headerlink" title="测试类及输出"></a>测试类及输出</h2><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">/**</span></span><br><span class="line"><span class="comment"> * <span class="doctag">@author</span> zhh</span></span><br><span class="line"><span class="comment"> * <span class="doctag">@description</span> 测试类</span></span><br><span class="line"><span class="comment"> * <span class="doctag">@date</span> 2020-02-04 23:57</span></span><br><span class="line"><span class="comment"> */</span></span><br><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">Test</span> </span>&#123;</span><br><span class="line"></span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">main</span><span class="params">(String[] args)</span> </span>&#123;</span><br><span class="line">        IGoods iGoods = <span class="keyword">new</span> Food(<span class="number">1</span>, <span class="string">"巧克力"</span>, <span class="number">9.9</span>);</span><br><span class="line"></span><br><span class="line">        String msg = String.format(<span class="string">"商品ID: %s, 商品名称: %s, 商品价格: %s"</span>,</span><br><span class="line">                iGoods.getId(), iGoods.getName(), iGoods.getPrice());</span><br><span class="line"></span><br><span class="line">        System.out.println(msg);</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<p>测试类的输出结果如下：</p>
<blockquote>
<p>商品ID: 1, 商品名称: 巧克力, 商品价格: 9.9</p>
</blockquote>
<h2 id="类结构图"><a href="#类结构图" class="headerlink" title="类结构图"></a>类结构图</h2><p>以上示例类的结构图如下所示</p>
<p><img src="https://oss.zhaohaihao.com/photo/design-patterns/ocp/ocp01.png?x-oss-process=style/style01" alt="image.png"></p>
<h1 id="需求变动"><a href="#需求变动" class="headerlink" title="需求变动"></a>需求变动</h1><p>假设现在超市进行一系列的商品促销活动，全场食品类的商品打6折，那我们如何来实现这个需求？</p>
<h2 id="方式一"><a href="#方式一" class="headerlink" title="方式一"></a>方式一</h2><p>针对上述的场景示例，我们想当然地会进行如下的改动，既快速又方便。</p>
<ul>
<li>首先在商品接口中新定义一个获取商品折扣价的方法。</li>
</ul>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">/**</span></span><br><span class="line"><span class="comment"> * <span class="doctag">@author</span> zhh</span></span><br><span class="line"><span class="comment"> * <span class="doctag">@description</span> 商品接口</span></span><br><span class="line"><span class="comment"> * <span class="doctag">@date</span> 2020-02-04 22:47</span></span><br><span class="line"><span class="comment"> */</span></span><br><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">interface</span> <span class="title">IGoods</span> </span>&#123;</span><br><span class="line">    <span class="comment">// 此处省略其余方法</span></span><br><span class="line">    </span><br><span class="line">    <span class="comment">// 新增 获取商品折扣价格 方法</span></span><br><span class="line">    <span class="function">Double <span class="title">getDiscountPrice</span><span class="params">()</span></span>;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<ul>
<li>然后在具体的商品子类当中去实现该方法。</li>
</ul>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">/**</span></span><br><span class="line"><span class="comment"> * <span class="doctag">@author</span> zhh</span></span><br><span class="line"><span class="comment"> * <span class="doctag">@description</span> 食品类</span></span><br><span class="line"><span class="comment"> * <span class="doctag">@date</span> 2020-02-04 23:42</span></span><br><span class="line"><span class="comment"> */</span></span><br><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">Food</span> <span class="keyword">implements</span> <span class="title">IGoods</span> </span>&#123;</span><br><span class="line">	<span class="comment">// 此处省略其余方法</span></span><br><span class="line">    </span><br><span class="line">    <span class="comment">// 子类实现该方法</span></span><br><span class="line">    <span class="function"><span class="keyword">public</span> Double <span class="title">getDiscountPrice</span><span class="params">()</span> </span>&#123;</span><br><span class="line">        <span class="keyword">return</span> getPrice() * <span class="number">0.6</span>;</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>


<p>上述简单的小改动，看样子确实是满足了我们的需求，但是仔细观察你会发现，这种改动不仅修改了接口，同时也对具体的实现类也进行了改动。</p>
<p>问题也就来了。假设我们商品分类有很多，那么所有的商品实现类都需要去实现接口新增的方法。而接口作为一种契约，是不应该经常变化的，它应该是稳定且可靠的。</p>
<h2 id="方式二"><a href="#方式二" class="headerlink" title="方式二"></a>方式二</h2><p>通过扩展新建一个食品的子类用来处理食品类打折，覆写食品类获取价格的方法，同时提供一个获取商品原价的方法。</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">/**</span></span><br><span class="line"><span class="comment"> * <span class="doctag">@author</span> zhh</span></span><br><span class="line"><span class="comment"> * <span class="doctag">@description</span> 食品打折类</span></span><br><span class="line"><span class="comment"> * <span class="doctag">@date</span> 2020-02-05 00:27</span></span><br><span class="line"><span class="comment"> */</span></span><br><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">FoodDiscount</span> <span class="keyword">extends</span> <span class="title">Food</span> </span>&#123;</span><br><span class="line"></span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="title">FoodDiscount</span><span class="params">(Integer id, String name, Double price)</span> </span>&#123;</span><br><span class="line">        <span class="keyword">super</span>(id, name, price);</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    <span class="function"><span class="keyword">public</span> Double <span class="title">getOriginPrice</span><span class="params">()</span> </span>&#123;</span><br><span class="line">        <span class="keyword">return</span> <span class="keyword">super</span>.getPrice();</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    <span class="meta">@Override</span></span><br><span class="line">    <span class="function"><span class="keyword">public</span> Double <span class="title">getPrice</span><span class="params">()</span> </span>&#123;</span><br><span class="line">        <span class="keyword">return</span> <span class="keyword">this</span>.getOriginPrice() * <span class="number">0.6</span>;</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<h3 id="测试类及输出-1"><a href="#测试类及输出-1" class="headerlink" title="测试类及输出"></a>测试类及输出</h3><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">/**</span></span><br><span class="line"><span class="comment"> * <span class="doctag">@author</span> zhh</span></span><br><span class="line"><span class="comment"> * <span class="doctag">@description</span> 测试类</span></span><br><span class="line"><span class="comment"> * <span class="doctag">@date</span> 2020-02-04 23:57</span></span><br><span class="line"><span class="comment"> */</span></span><br><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">Test</span> </span>&#123;</span><br><span class="line"></span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">main</span><span class="params">(String[] args)</span> </span>&#123;</span><br><span class="line">        IGoods iGoods = <span class="keyword">new</span> FoodDiscount(<span class="number">1</span>, <span class="string">"巧克力"</span>, <span class="number">9.9</span>);</span><br><span class="line"></span><br><span class="line">        FoodDiscount foodDiscount = (FoodDiscount) iGoods;</span><br><span class="line"></span><br><span class="line">        String msg = String.format(<span class="string">"商品ID: %s, 商品名称: %s, 商品原价: %s, 商品折后价: %s"</span>,</span><br><span class="line">                foodDiscount.getId(), foodDiscount.getName(), foodDiscount.getOriginPrice(), foodDiscount.getPrice());</span><br><span class="line"></span><br><span class="line">        System.out.println(msg);</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<p>测试类的输出结果如下：</p>
<blockquote>
<p>商品ID: 1, 商品名称: 巧克力, 商品原价: 9.9, 商品折后价: 5.94</p>
</blockquote>
<h3 id="类结构图-1"><a href="#类结构图-1" class="headerlink" title="类结构图"></a>类结构图</h3><p>方式二改动后类的结构图如下所示</p>
<p><img src="https://oss.zhaohaihao.com/photo/design-patterns/ocp/ocp02.png?x-oss-process=style/style01" alt="image.png"></p>
<h3 id="好处"><a href="#好处" class="headerlink" title="好处"></a>好处</h3><ul>
<li>不修改底层的基类和接口，防止了风险的扩散。</li>
<li>通过继承基类，使得对扩展开放，对于修改接口和基类是关闭的。</li>
</ul>
<h1 id="优点"><a href="#优点" class="headerlink" title="优点"></a>优点</h1><p>提高软件系统的可复用性和可维护性</p>
<h1 id="参考"><a href="#参考" class="headerlink" title="参考"></a>参考</h1><ul>
<li>《Head First 设计模式》</li>
<li>《大话设计模式》</li>
</ul>

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